This invention relates generally to horizontal phase locked loop (PLL) systems and particularly to an improved PLL system for use in conjunction with a video cassette recorder (VCR).
Horizontal PLL systems are well-known in the art and comprise a horizontal phase detector, a low pass filter, an error amplifier for supplying a control signal to the horizontal oscillator circuit, and a divider network. The divider network supplies a signal that is representative of the horizontal oscillator signal back to the horizontal phase detector for comparison with the input horizontal sync signal. The horizontal oscillator in turn controls horizontal deflection of an electron beam in a cathode ray tube (CRT) in conjunction with a vertical deflection system for generating a raster on the faceplate of the CRT. The electron beam is modulated with video information to develop a video picture. As is well-known, the design of the horizontal PLL circuit represents a compromise between achieving good lock-in capability for weak signals and minimizing horizontal jitter.
When a VCR is used as a television signal source, horizontal PLL circuits that are designed for optimum operation with conventional television signals may experience difficulties due to variation in the time base of the VCR signal. Stretching of the tape is one example of how such a time base error is introduced into the VCR signal output. The result of such a time base error is that the PLL circuit may not lock in immediately after the vertical blanking interval. This shows up as a "hook" or bending at the top of the raster, with the severity of the hook varying with the amount of time base error in the tape source. Such a hook in the display can be very disturbing to a viewer and is clearly not desirable.
One means for eliminating such a hook is to decrease the PLL response time, that is, increase the gain of the PLL circuit, when the receiver is used with signal sources that are prone to time base errors. A viewer-controlled switch may be provided for this purpose. Since VCR signals are generally carried on either television VHF channels 3 or 4, one prior art solution automatically changes the horizontal PLL response time when either channel 3 or channel 4 is selected on the television receiver. While these prior art solutions are effective, they either entail additional cost or are inconvenient. Accordingly, there is a need in the art for a simple, automatic correction to compensate for VCR signal time base errors.